PROJECT SUMMARY/ABSTRACT Congenital heart disease (CHD) is one of the most common congenital disorders, affecting about 1% of all live births. More than half of children with moderate or severe CHD have neurodevelopmental disabilities (NDD) that persist into later life. Quantitative methods that can objectively identify subjects at high risk for NDD as early as possible are needed to allow for characterization of the mechanisms underlying NDD and monitor the success of potential therapeutic interventions. Brain magnetic resonance imaging (MRI) studies provide evidence for a prenatal origin of NDD by detecting reduced global brain volumes and gyrification in fetuses with CHD. However, these whole-brain measures do not provide insight into regional brain vulnerabilities or second trimester differences in brain development. In addition, no studies have yet explored the potential role of genetic variants and attempted to disentangle the relative contributions of genetic and hemodynamic factors on prenatal brain development in CHD. Since patients with single-ventricle (SV) CHD suffer high rates of NDD and show both severe in utero reduction in oxygenated cerebral blood supply and frequent damaging genetic variants, fetal SV CHD cohort is an ideal group in which to explore markers of altered early brain development influenced by genetic and/or cerebral hemodynamic factors. This study will examine sulcal patterns and regional cortical growth (thickness and surface area) to indicate the effects of genetic variants and altered cerebral hemodynamics respectively using a large dataset of retrospective and prospective longitudinal MRIs from the second trimester to birth with three time points in 175 SV CHD and 260 typically developing (TD) subjects. Sulcal pattern development will be compared between SV CHD and TD subjects and correlated with rare damaging variants in high heart expression (HHE) genes and presence of neuroprotective APOE allele in SV CHD. We will also develop surface-based regional analysis of fetal cortical thickness and surface area. Regional cortical thickness and surface area will be compared between SV CHD and TD subjects and correlated with different types of cerebral blood flow in SV CHD. We hypothesize that sulcal pattern alterations will be detected in SV CHD in the second trimester and more abnormal in SV CHD subjects with rare damaging variants in HHE and/or neuroresilience APOE genes. Differences in cortical thickness and surface area between SV CHD and TD subjects may be regionally inhomogeneous and more severe in the frontal regions. Regional cortical growth will be more reduced in SV CHD subjects with aortic outflow obstruction and retrograde aortic arch flow compared to the subjects with pulmonary obstruction with anterograde arch flow or unobstructed outflow tracts. This project would allow early identification of the relative roles of altered genetics and cerebral perfusion and lay the foundation for early selection and optimization of individual therapy and deciding the efficacy of the interventions in CHD.